Submarine communications antenna system



Sept. 8, 1970 r. K. ALBEE SUBMARINE COMMUNICATIONS ANTENNA SYSTEM FiledJune 10, 1966 RADIO 7 RECEIVER FIG?) M i M QM \& m z m m w E D.. P RE WW W mm .E\ NA W 17H IZN WK. m m W8 N MHWN 2 M v 2 m H MT\ UW M TM I T m;l i m Wmmm m m w m 0 a m M M I E 4 m M /W m m L m F Tm /w Q m /W M t mmm mmmm m mwmw m m 25: z 5&8 @352 BY M,

FREQ. IN KHZ ATTORNEYS United States Patent Oihce 3,528,014 PatentedSept. 8., 1970 3,528,014 SUBMARINE COMMUNICATIONS ANTENNA SYSTEM ThomasK. Albee, Lisle, 11]., assignor to The Bunker- Ramo Corporation, acorporation of Delaware Filed June 10, 1966, Ser. No. 556,790 Int. Cl.H04]: 1/16 US. Cl. 325427 18 Claims ABSTRACT OF THE DISCLOSURE Methodand apparatus for the cancellation of longitudinal non-Gaussian noise ina submarine antenna system and for increasing the Q of an antenna bycompensating for the radiation resistance term of the inherent circuitlosses by the coupling of a negative impedance means to the antennacircuit. The antenna system may provide a substantially constantbandwidth over a wide range of selected operating frequencies within thelow frequency bands by the simultaneous tuning of the loop antennasystem and the adjusting of the negative impedance means.

This invention relates generally to antenna systems and moreparticularly to methods and apparatus for increasing the signal-to-noiseratio in antenna systems of the type intended for use in the lowfrequency bands.

Radio communication for submarines is often conducted at operatingsignal frequencies within the LF. (low freqeuncy) and V.L.F. (very lowfrequency bands), referred to collectively herein as the low frequencybands, as underwater transmission of radio signals is particularlypractical within these frequency bands. Submarine radio systems, forexample, are usually operated at frequencies within a band ofapproximately three kilocycles to three hundred kilocycles, withoperation at a frequency of twenty kilocycles being common.

Such low frequency radio communication systems generally utilize fixedor rotatable loop antennas which have been found to provide the bestavailable performance characteristics in a Water environment withrespect to signal-to-noise ratio, directivity patterns, and signalstrength. A variable capacitive reactance is normally connected acrossthe loop antenna as a tuning impedance to allow selective tuning of theantenna for operation at more than one frequency. Because submarinecommunication systems operate at relatively low frequencies, a loopantenna and variable capacitance having relatively large inductance andcapacity with attendant large physical dimensions are generallyrequired.

In the underwater communication systems heretofore known, andparticularly in underwater V.L.F. radio receiving systems, the largeimpedances of the loop antenna and the variable tuning capacitance haveintroduced substantial component noise and inherent circuit losses whichtend to decrease the sensitivity of the receiving system. This decreasein sensitivity is particularly troublesome in the low frequency bandsbecause of the high level of atmospheric noise and man-made interferenceencountered there. Accordingly, it has long been desired to improve thesignal-to-noise ratio of such underwater radio receiving systems.

In many instances an increase in the effective height of the antennasystem will increase the amplitude of the input signal to the receiver,and therefore increase the receiver signal-to-noise ratio. However, itis usually difficult to substantially increase the effective height ofthe antenna system on a submarine, due to practical limitations.

The introduction of an ordinary pre-amplifier between the antenna systemand the receiver input, although tending to increase the magnitude ofthe signal to the receiver, would also increase the magnitude of thenoise present in the antenna circuit and could also introduce additionalcomponent noise originating in the pre-amplifier itself. Similarly, theuse of conventional passive filter networks, although eliminating someof the noise from the receiver system, is not particularly effective inincreasing the signal-to-noise ratio because of the substantial reduction in signal amplitude caused by the filter network.

Accordingly, it is an object of the present invention to provide amethod of increasing the signal-to-noise ratio in tuned loop antennasystems of the type adapted for use in the low frequency bands.

It is an additional object of the present invention to provide animproved loop antenna system of the type adapted for use in the loWfrequency bands, which antenna system presents a smaller operatingbandwidth and a greater output signal amplitude than similar antennasystems heretofore known.

It is a further object of the present invention to obtain theaforementioned reduction of operating bandwidth and increase in outputsignal amplitude of tuned loop antenna systems by the insertion in suchantenna systems of circuit means having relatively small physical sizeand weight.

A further object of the present invention is to use a negative impedancenetwork to improve the sensitivity and selectivity of a low frequencytuned loop antenna system.

Briefly, the present invention contemplates the coupling of negativeimpedance circuit means with a tuned loop antenna system to effectivelycancel a substantial portion of the inherent circuit losses in theantenna system. The resultant signal provided by the antenna system andthe negative impedance means is fed to the active input stage of thereceiver. Since the operating bandwith and the output signal amplitudeof the tuned antenna system depend upon the magnitude of the inherentcircuit losses in the system, a reduction in the magnitude of suchcircuit losses will provide a reduced bandwith and an increased outputsignal amplitude. The increased signal amplitude coupled with a reducednoise level produced by the reduction in bandwidth consequently servesto increase the signal'to-noise ratio of the antenna system. The presentinvention also optionally contemplates the provision of means for tuningthe tuned antenna system to selected operating frequencies within thelow frequency bands and simultaneously adjusting the magnitude of thenegative impedance presented by the negative impedance circuit means tothereby maintain the operating bandwidth of the antenna system nearlyconstant over a wide range of selected operating frequencies.

The invention and its many advantages will be further understood byreference to the following detailed description illustrated in theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of a tuned antenna system constructed inaccordance with the teachings of the present invention;

FIG. 2 is the equivalent circuit diagram of the tuned antenna systemshown in FIG. 1 of the drawings;

FIG. 3 is a graphic showing of the improvement in the quality factor orQ of a tuned antenna system obtained through the use of the presentinvention;

FIG. 4 is a graphic showing of the improvement in output signalamplitude of a tuned antenna system obtained through the use of thepresent invention; and

FIG. 5 is a schematic diagram of a tuned antenna system constituting analternate embodiment of the invention.

Referring now to FIG. 1 of the drawings, there is shown an antennasystem including a loop antenna which is connected to a complete radioreceiver system 12 by means of leads 14-. Antenna 10 may take any of anumber of conventional loop antenna configurations, but will generallycomprise a large number of wire turns wound around a relatively largesupport in order to provide a sufiicient effective antenna height forsignal reception in the low frequency bands. In submarine signallingapplications, for example, antenna 10 will generally be located at asubstantial distance from receiver 12, and therefore leads 14 may extendfor lengths up to several hundred feet. At the wavelengths involved,however, such a line is electrically very short, although it mayconstitute a material resistance.

Connected in parallel with the loop antenna 10 is a variable capacitance16 which may be selectively varied in magnitude in order to tune theinductive antenna 10 to receive a selected operating frequency withinthe low frequency bands the antenna is adapted to receive. Althoughshown as connected adjacent loop 10, capacitor 16 may be connected atother positions along leads 14. As previously mentioned, althoughfrequencies within the band of three kilocycles to three hundredkilocycles are commonly used for underwater signalling, frequencieswithin a band of five kilocycles to one hundred kilocycles are ofprimary interest in the disclosed system. In practice, because of thelarge size of antenna 10, capacitor 16 usually comprises a plurality ofsmall capacitors provided with a ganged switching arrangement to providesufiicient capacitance for selective tuning of the antenna system.Again, because of the large size of antenna 10 and additionally, becauseof the required plurality of capacitors, large inherent circuit losseswhich are primarily resistive in nature are introduced into the system.These losses tend to attenuate the received signal and increase thebandwidth of the system, thereby adversely affecting the signal-tonoiseratio of the system output signals.

In accordance with the teachings of the present invention, a negativeimpedance circuit 18 is coupled with the loop 10 and capacitor 16 tosupply the improved resultant signal to the radio receiver 12. Asillustrated, the negative impedance circuit 18 is connected in parallelwith the antenna system 10, 16 where it serves to effectively cancel aportion of the inherent circuit losses present in the antenna system.The negative impedance circuit may be coupled to the system adjacent theloop 10, the receiver 12, or as shown at an intermediate point alongleads 14. In a manner hereinafter described in greater detail, theinsertion of the negative impedance circuit causes a reduction of theoperating bandwidth of the antenna system and an increase in theamplitude of the signal supplied to the input of radio receiver 12.Since the reduc tion in bandwidth of the system reduces the receivednoise level and compensates the increase in effective resistance atresonance in respect to thermal noise level of the loop and capacitor,and since the amplitude of the output signal is increased, thesignal-to-noise ratio of the resultant signal available to radioreceiver 12 is greatly increased.

With respect to negative impedance circuit 18, it may be observed thatit has long been known to be possible to convert positive impedancesinto effective negative impedances by coupling the output of anamplifier back into its own input, and many circuits have been designedto obtain this result. While any one of a plurality of wellknownnegative impedance circuits may be utilized in the present invention, aknown type of transistor negative impedance circuit has beenschematically illustrated in FIG. 1. As will be understood by thoseskilled in the art, circuit 18 is a push-pull type of negative impedanceconverter which employs cross-coupling feedback between twointerconnected transistors 20 and 22. The two transistors and theirassociated circuit components are symmetrically disposed and commonlyconnected so that the output terminal of each transistor is coupled tothe common connection of the other, thereby providing phase invertingfeedback. The resulting magnitude of negative impedance presented bycircuit 18 depends upon the portion of the voltage supplied across thecircuit which is cross coupled between the transistors, and accordingly,circuit 18 includes several variable bias adjustments for adjustment ofthe negative impedance. For a detailed description of negative impedancecircuit 18, reference may be had to the article by J. G. Linvillappearing in 41 I.R.E. Proceedings, 726-729, June 1953.

Additionally, a variable load impedance 24 is provided in circuit 18 toallow the magnitude of the negative impedance to be variedsimultaneously with the tuning of the antenna circuit. To this end, amechanical coupling 26 may be employed to interconnect the variablecapacitance 16 with the impedance 24 in order to permit simultaneousadjustment of the two variables. As will become more apparent from thesubsequent description, such a simultaneous adjustment will provide anequalized or substantially constant improved bandwidth for the antennasystem as the tuned operating frequency of the antenna is changed. Itwill be understood, however, that the mechanical drive ratio betweencapacitance 16 and impedance 24 will usually be a complex functionrather than a directly proportional relationship.

The operation of the present invention may best be understood byreference to FIG. 2 of the drawings, wherein the equivalent circuitdiagram of the loop antenna system shown in FIG. 1 is illustrated. Theloop antenna 10 in FIG. 2 is generally represented by an equivalentinductance L and the variable tuning capacitor 16 is represented at onetuned position by an equivalent capacitance C. The total equivalentseries resistance R of the tuned antenna system is the resultant of thesum of all resistance losses in the circuit, such as the winding andcable resistance, the radiation resistance of the loop antenna, and theequivalent series resistance of the antenna core losses. As previouslydescribed, the loss resistance R is usually relatively large for tunedloop antenna systems adapted for use in the low frequency bands and thusnormally tends to decrease the sensitivity of such systems.

The output impedance Z presented by a conventional loop antenna systemwhen tuned to resonance at an operating frequency may be represented as:

RT (1) where X is the inductive reactance of the loop antenna and X isthe capacitive reactance of the tuning capacitor.

The quality factor, or Q of the equivalent tuned antenna system shown inFIG. 2 is:

output impedance Z of the tuned antenna system will become:

It will be seen from an inspection of Equation 3 that if the absolutemagnitude of the negative impedance Z is greater than the absolutemagnitude of impedance Z the antenna system will be stable and Z willhave a greater magnitude than Z Hence, if a negative impedance Z of apredetermined magnitude is inserted across a tuned antenna systemaccording to the present invention, the effective resultant impedance ofthe tuned antenna system will be substantially increased from its normalmagnitude.

Further, as may be seen from Equation 1, an increase in the outputimpedance Z of the tuned antenna system will result in a correspondingdecrease in the effective total equivalent series loss resistance R ofthe tuned antenna circuit. Additionally, as may be seen in Equation 2,the quality factor Q of the antenna system will increase and theeffective bandwidth Af of the system will decrease because of thereduction in magnitude of R Since the output signal, or open circuitoutput voltage, of the tuned antenna system is directly proportional tothe magnitude of the Q in the system, an increase in Q will alsoincrease the output signal amplitude presented by the tuned antennasystem to the radio receiver 12. This increase in output signalamplitude coupled with the decrease in noise level brought about by thereduction in operating bandwidth of the system vastly improves thesignal-to-noise ratio of the antenna system output.

As an example of the improvement provided by the present invention,consider a tuned antenna system as shown in FIGS. 1 and 2 of thedrawings having the following values before the insertion of a negativeimpedance:

OhmS f =20K hertz Q=40 Therefore, from Equations 1 and 2:

R 1.5 ohms Af=500 hertz Z =2400 j60 ohms.

If a negative impedance -Z having a value of 2670+ 173.4 ohms isinserted across the tuned antenna in parallel with the variablecapacitance, as illustrated in FIG. 2, then from Equation 3:

As this value of Z is now the effective resultant output impedancepresented to the input of the receiver, the effective series lossresistance R and bandwidth Af of the antenna system are subtantiallyreduced to:

R =.15 ohm Af=50 hertz Similarly, from Equation 2, the Q of the circuitwill be increased by a factor of to 400, thereby also effecting acorresponding increase in the open circuit resultant output voltage ofthe system. The above-described substantial improvements in theoperating characteristics of the tuned antenna system are achievedwithout the insertion of significant additional noise. In practice, theeffective tuned bandwidth of present loop antennas may be reduced by afactor of 40 which results in an improved db down signal-tonoisethreshold sensitivity of 32 db. This figure is highly conservativebecause it does not take into account the further reduction inatmospheric noise level obtained by the reduced bandwidth.

FIG. 3 of the drawings illustrates the improvement in the quality factoror Q of a tuned antenna system obtained by the use of a negativeimpedance circuit as described herein. The readings were taken for aloop antenna with athwart winding and 227 feet of cable leading to thetest equipment installation. The field was supplied by an overhead testwire energized at the signal voltage of 0.08 volt and arrangedperpendicularly to the center line of the loop winding. FIG. 4 of thedrawings shows the corresponding improvement in output voltage of thesame antenna system with the same test parameters.

Although the improved antenna system of the present invention has thusfar been described and illustrated with the negative impedance circuitconnected in parallel circuit across the output of the tuned antenna, itshould be pointed out that similar improvements in antenna systemparameters may be obtained by inserting a suitably designed negativeimpedance circuit 18 in series circuit between capacitor 16' and radioreceiver 12', as shown in FIG. 5 of the drawings. In this configuration,the receiver may preferably comprise as an initial stage an isolatingamplifier so that its conventional active circuitry will not be subjectto the negative impedance. In this case, however, since the effectiveterminal impedance Z of the system is:

Z2'=Z1' ZNI where Z is the equivalent impedance of the antenna 10' andtuning capacitor 16 and Z is the impedance of the negative impedancecircuit 18', the absolute magnitude of the negative impedance Z must beless than the absolute magnitude of the equivalent impedance Z of theantenna circuit to avoid instability. The parallel circuit arrangementof FIG. 1 of the drawings is somewhat preferable to the series circuitarrangement of FIG. 5 because in the former arrangement the negativeimpedance is in parallel with the antenna terminals and therefore offersgreater reliability and fail-safe operation.

Although not shown in the drawing, series negative impedance 18 couldalternatively be placed between loop 10' and capacitor '16, in whichcase the latter would be directly connected to the conventional inputcircuitry of a receiver such as 12 of FIG. 1. As in the otherembodiments disclosed, the selected negative impedance employed is of avalue to compensate most but not all the resistive loss of the LC systemof 10-16' at resonance. Consequently, the thermal noise developed in thetuned system, which establishes the value of the smallest detectablesignal for the receiving system, is not degraded because, as Q isincreased, the decrease in bandwidth compensates the increase in QXAlthough a preferred embodiment of the invention has been describedherein, it is believed to be obvious that many changes could be made inthe disclosed method and apparatus without departing from the scope ofthe invention. Accordingly, it is intended that all matter contained inthe above description or shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An antenna system adapted for use in the low frequency bandscomprising:

tuned antenna means adapted to receive radio signals in the lowfrequency bands, said tuned antenna means comprising: loop antennameans, capacitor means coupled across said loop antenna means for tuningthe antenna system to a selected operating frequency within said lowfrequency bands, and antenna output means coupled to radio receivercircuit means, the operating bandwidth of said tuned antenna means andthe amplitude of the output signal appearing at said output means beingdependent upon the inherent circuit losses within said tuned antennameans; and

negative impedance circuit means coupled in series circuit with saidantenna output means and said radio receiver circuit means for insertinga negative im pedance of an absolute magnitude less than the absolutemagnitude of the equivalent impedance of said tuned antenna meanswithout producing an unstable condition therein, to thereby reduce theoperating bandwidth of the antenna system and increase the amplitude ofthe output signal therefrom.

2. An antenna system as claimed in claim 1 wherein said capacitor meansand said negative impedance circuit means are adjustable to vary theselected operating frequency of the antenna system and wherein saidantenna system further comprises:

means coupled to both said capacitor means and said negative impedancecircuit means for simultaneously adjusting said capacitor means and saidnegative impedance means to provide a substantially constant bandwidthfor said antenna system over a wide range of selected operatingfrequencies within said low frequency bands.

3. An antenna system as claimed in claim 1 wherein said negativeimpedance means includes a plural transistor balanced network wherebythe longitudinal noise is cancelled.

4. An antenna system as claimed in claim 1 wherein said tuned antennameans is immersed in a conducting medium.

5. An antenna system as claimed in claim 4 wherein said negativeimpedance means includes a plural transistor balanced network wherebythe longitudinal noise is cancelled and wherein the conducting medium issea water.

6. An antenna system adapted for use in the low frequency bandscomprising:

tuned antenna means adapted to receive radio signals in the lowfrequency bands, said tuned antenna means comprising: loop antennameans, capacitor means coupled across said loop antenna means for tuningthe antenna system to a selected operating frequency within said lowfrequency bands, and antenna output means coupled to radio receivercircuit means, the operating bandwidth of said tuned antenna means andthe amplitude of the output signal appearing at said output means beingdependent upon the inherent circuit losses within said tuned antennameans; and

negative impedance circuit means coupled in parallel circuit with saidtuned antenna means for inserting a negative impedance of an absolutemagnitude greater than the absolute magnitude of the equivalentimpedance of said tuned antenna means without producing an unstablecondition therein, to thereby reduce the operating bandwidth of theantenna system and increase the amplitude of the output signaltherefrom.

7. An antenna system as claimed in claim 6 wherein said capacitor meansand said negative impedance circuit means are adjustable to vary theselected operating fre quency of the antenna system and wherein saidantenna system further comprises:

means coupled to both said capacitor means and said negative impedancecircuit means for simultaneously adjusting said capacitor means and saidnegative impedance means to provide a substantially constant bandwidthfor said antenna system over a wide range of selected operatingfrequencies within said low frequency bands.

8. An antenna system as claimed in claim 6 wherein said negativeimpedance means includes a plural transistor balanced network wherebythe longitudinal noise is cancelled.

9. An antenna system as claimed in claim 6 wherein said tuned antennameans is immersed in a conducting medium.

10. An antenna system as claimed in claim 9 wherein said negativeimpedance means includes a plural transistor balanced network wherebythe longitudinal noise is cancelled, and wherein the conducting mediumis sea water.

11. The method of increasing the output signal-to-noise ratio of tunedloop antenna systems of the type intended for use in the low frequencybands wherein the operating bandwidth and output signal amplitude of theantenna system depend upon the inherent circuit losses therein,comprising the steps of:

compensating for a substantial part, but less than all,

8 of said inherent circuit losses by the coupling in series circuit withthe antenna system a balanced negative impedance means having anabsolute magnitude less than the absolute magnitude of the equivalentimpedance of the antenna system to thereby reduce the operatingbandwidth of the system and increase the amplitude of the output signaltherefrom, and feeding the resultant signal to radio circuit means towhich said system is adapted to be coupled.

12. The method of claim 11 further comprising the step of tuning thesaid loop antenna system to selected operating frequencies within thesaid low frequency bands and simultaneously adjusting said negativeimpedance means to provide a substantially constant bandwidth over awide range of selected operating frequencies within said low frequencybands.

13. The method of claim 11 wherein the antenna of the loop antennasystem is immersed in a conducting medium.

14. The method of claim 13 further comprising the step of tuning thesaid loop antenna system to selected operating frequencies within thesaid low frequency bands and simultaneously adjusting said negativeimpedance means to provide a substantially constant bandwidth over awide range of selected operating frequencies within said low frequencybands.

15. The method of increasing the output signal-to-noise ratio of tunedloop antenna systems of the type itended for use in the low frequencybands and wherein the operating bandwidth and output signal amplitude ofthe antenna system depend upon the inherent circuit losses therein,comprising the steps of:

compensating for a substantial part, but less than all,

of said inherent circuit losses by the coupling in parallel circuit withthe said antenna system of negative impedance means having an absolutemagnitude greater than the absolute magnitude of the equivalentimpedance of the antenna system to thereby reduce the operatingbandwidth of the system and increase the amplitude of the output signaltherefrom, and

feeding the resultant signal-to-radio circuit means to which said systemis adapted to be coupled.

16. The method of claim 15 further comprising the step of tuning thesaid loop antenna system to selected operating frequencies within thesaid low frequency bands and simultaneously adjusting said negativeimpedance means to provide a substantially constant bandwidth over awide range of selected operating frequencies within said low frequencybands.

17. The method of claim 15 wherein the antenna of the loop antennasystem is immersed in a conducting medium.

18. The method of claim 17 further comprising the step of tuning thesaid loop antenna system to selected operating frequencies within thesaid low frequency bands and simultaneously adjusting said negativeimpedance means to provide a substantially constant bandwidth over awide range of selected operating frequencies within said low frequencybands.

OTHER REFERENCES K. Iizuka: IEEE-Transactions on Antennas andPropagation, VAP-l3 n. 1, January 1965, pp. 7-20.

KATHLEEN H. CLAFFY, Primary Examiner C. W. JIRAUCH, Assistant Examiner

